The cellular changes associated with the development and maturation of the chick trigeminal ganglion can be summarized as follows: (1) dual cellular origin from neural crest and placodal ectoderm material; (2) a transitory topograhical arrangement of small- and large-sized cells; (3) an apparently random distribution of small and large neurons; and (4) a final random arrangement of dark and light neurons. Although it has been demonstrated that the neural crest cells were the precursors of the small cells and that the placodal ectoderm cells were the precursors of the large cells, no such correlation has been investigated concerning the dark and light trigeminal neurons. It is the specific aim of this work to investigate the relationship between the topographically arranged small and large cells within early developing chick trigeminal ganglia and the dark and light neurons observed within post-hatching and adult ganglia. Due to differences in nuclear morphology, the quail cell is quite easily distinguished from the chicken cell. Isotopic and isochronic grafts of both quail neural crest material and placodal ectoderm material will be orthotopically placed into chick embryos of the same age. Chorioallantoic membrane grafts and the horseradish peroxidase retrograde tracer technique will also be used to further define this relationship. In this manner, the migration of the quail cells and their fate in relation to small and large cells and dark and light cells can be investigated.